Every year, diarrhea causes
around five million fatalities worldwide. Most people die due to pathogenic
microorganisms, such as bacteria or viruses, which were ingested into the
gastro-intestinal tract through contaminated drinking water or food.
Determining which bacterium is causing the illness in those cases is
sometimes very complex. In cooperation with Chilean researchers, scientists
from the Helmholtz Centre for Infection Research (HZI) in Braunschweig,
Germany, have now developed a fine-tuned diagnostic method. It is based on
detecting short, repetitive DNA segments in the genome of bacteria. Every
single bacterial strain has such characteristic repeats.

“With this method we are
able to identify bacterial strains as well as clarify their genetic
relationships. Furthermore, we can show how new pathogenic variants develop,”
says Manfred Höfle, researcher at the HZI. The results have now been
published in the current issue of the scientific journal Applied and Environmental
Microbiology. The work is part of the two European Union funded
projects “Healthy Water” and “AQUA-chip”. Manfred Höfle is coordinator of
both projects that deal with various aspects of the microbiological safety of
both, drinking water and sea water.

Various bacteria that live
in drinking water or sea water can cause severe human diseases. One of them
are vibrios: its species Vibrio
cholerae is more commonly known as the causative agent of Cholera
that spread in Europe until the 20th century. Interestingly, not all Vibrio cholerae strains
are pathogenic to humans. Only those strains cause severe diarrhoea known as
Cholera that produce a certain bacterial toxin which attacks the intestinal
wall. A less known, though also dangerous member of the genus Vibrio, is
Vibrio parahaemolyticus. It is a highly contagious pathogenic germ with only a
dozen ingested bacteria causing severe diarrhoea. This strain is a threat for
the pacific region and reached the east coast of the United States in the
21st century. Since the end of the 1990s, Vibrio parahaemolyticus epidemics
have led to thousands of cases of illness in Chile. In the future, due to
ballast water or climate change, the species may also gain importance in
Europe. As in the Cholera bacterium, various Vibrio parahaemolyticus strains
exist with varying infectivity. Distinguishing those strains has been a
challenge until now.

The newly developed method
makes it now possible to characterize and distinguish hundreds of bacteria
strains in a short time. The method is based on the existence of short,
repetitive DNA segments in the genome of all living species. As in a tandem
bike, those segments are lined up on the DNA strand, called “tandem repeats”.
They are characteristic for every bacterial strain. To identify a certain
strain, the HZI researchers use short DNA fragments, marked with certain dyes.
Each dyed DNA fragment recognizes a single tandem repeat, binding at it. As a
result, the researchers receive, for example, six red fragments binding a
tandem of six repetitions. Then, the researchers analyzed the tandem repeats
marked with dyed fragments: Every bacteria strain differs in pattern and size
of the measured tandem repeats.

“With this method, we are
able to differentiate more then 120 Vibrio
parahaemolyticus strains,” says Manfred Höfle. This is important
for infectious diseases in which it is necessary to know which strain is the
causative agent. Further information are whether it is just one or more
strains and where they derive from. The latter can help to prevent spreading
of the disease with corresponding sanctions. “The intake of Vibrio parahaemolytics
often occurs through raw clams that have filtered contaminated sea water.
With this method, we are able to say from which clam species the germ
originates.” The new technique can also be used to characterize other
bacterial pathogens and to investigate how pathogenic bacteria evolve in the
environment. “Hereby, this high resolution method makes an important
contribution towards a fast and precise recognition of microbial pathogens
with pandemic potential.”